PTO shafts (or simply "PTOs") are used on agricultural vehicles such as tractors to provide power for equipment or implements such as combines, mowers and spreaders. As the use of PTOs developed, most tractor manufacturers standardized on 1000 RPM and 540 RPM PTOs. This standardization involved the use of a common size shaft and spline arrangement for each RPM rating. When the shaft sizes were standardized years ago, tractors had relatively low horsepower (e.g., 30 to 50 horsepower). Accordingly, the torque output of a PTO was limited by the horsepower of the tractor. Modern tractors commonly have horsepower ratings in excess of 100 horsepower. However, the shaft sizes for PTOs have not changed due to the need to maintain compatibility with older equipment and maintain the standardization for PTOs. Thus, the torque output of PTOs for many modern tractors is no longer limited by the tractor horsepower. Rather, the torque output is limited by the strength of the PTO and the failure thereof. For very high horsepower tractors (e.g., over 130 horsepower), manufacturers have eliminated the 540 RPM PTO. Due to the gear reduction required to achieve a PTO speed of 540 RPM at engine idle, the very high horsepower tractors can apply a torque to the 540 RPM PTO in excess of that required for the PTO toil. In addition to causing PTO failures, the torque produced by the higher horsepower tractors also can accelerate equipment attached to the respective PTO at a rate which can damage the equipment.
Excessive acceleration of (or application of torques to) a PTO is of particular concern during the process of engagement of the PTO from a standstill or zero angular velocity state to a "lock-up" state, at which the PTO has an angular velocity equaling that of the engine (or, assuming various gear reductions, etc., an angular velocity that is an appropriate fraction or multiple of the angular velocity of the engine). Relevant components associated with this process of engagement of a PTO are shown in FIGS. 1 and 2 (prior art). FIG. 1 shows, in simplified form, a conventional (exemplary) arrangement for transmitting power from an engine 2 (of an agricultural vehicle) to a PTO 1. As shown, PTO 1 is capable of receiving power from engine 2 by way of a PTO clutch 3. PTO clutch 3 is capable of transmitting power from an input shaft 4, which receives power from engine 2, to an output shaft 10, which is in turn typically coupled to PTO 1 by way of one or more gears (not shown). The amount of power transmitted from engine 2 to PTO 1 depends upon whether PTO clutch 3 is engaged (i.e., whether plates within the clutch have been compressed sufficiently to allow the clutch to transmit torque) and, once the clutch has been engaged, upon the degree of hydraulic fluid pressure applied to the clutch, which determines the amount of torque that the clutch may transmit from input shaft 4 to the PTO via output shaft 10.
PTO 1 may be coupled, by way of a coupler 15, to an implement input shaft 5 (supported by an implement attached to the agricultural vehicle). Typically, implement input shaft 5, which is for receiving power from PTO 1, is in turn coupled to an implement output shaft 13 for transmitting the power to attached equipment supported by the implement. In certain embodiments, implement input shaft 5 may be coupled to implement output shaft 13 by way of an over-running clutch 6. Over-running clutch 6 allows implement input shaft 5 (and PTO 1) to transmit power to implement output shaft 13 but also allows the output shaft to continue to rotate freely when the input shaft no longer is rotating. As shown in FIG. 2, an exemplary over-running clutch 6 includes an arrangement in which an output 7 attached to output shaft 13 concentrically surrounds an input 8 attached to input shaft 5. Input 8 transmits power to output 7 only when spring-actuated locking pins 9 are fully extended into two locking grooves or notches 11 and when input 8 receives power (from PTO 1) causing the input to rotate in a counter-clockwise direction relative to output 7, in which case the input is coupled to the output. In other circumstances, such as when locking pins 9 are not fully extended into locking notches 11 (as shown), or when output 7 rotates in a counter-clockwise direction relative to input 8 (e.g., when no power is being transmitted from engine 2 but when output shaft 13 nonetheless is rotating in a counter-clockwise direction), output 7 freely rotates with respect to input 8 and effectively no power is transmitted between the two elements.
As shown in prior art FIG. 3A, PTO 1 experiences a rapid change in angular velocity during the PTO engagement process once PTO clutch 3 has been engaged such that power is transmitted from engine 2 to the PTO (e.g., after a time t.sub.1). In order to control PTO acceleration during this process of engagement of PTO 1, Case Corporation has developed a PTO clutch control system that monitors the angular velocities of input shaft 4 and output shaft 10 and controls the acceleration of PTO 1 based upon these measured velocities, as described in U.S. Pat. No. 5,494,142 to Kale and incorporated by reference herein. Based upon the monitored speeds of input shaft 4 and output shaft 10, the clutch control system calculates a desired acceleration for PTO 1 and also repeatedly calculates an actual acceleration of the PTO. The desired acceleration is calculated as the ratio of the angular velocity of input shaft 4 (or a quantity directly related to the engine speed of the agricultural vehicle) to a predetermined amount of time (shown in FIG. 3A as the time interval between a time t.sub.3 and a time t.sub.1), and is only calculated once. That is, only one calculated value of the desired acceleration is utilized by the clutch control system throughout the PTO engagement process. The actual acceleration is calculated as the ratio of the change in angular velocity of output shaft 10 (or a quantity related to the speed of PTO 1) during a particular time interval (the time between two velocity measurements) divided by the time interval. The predetermined amount of time with respect to the desired acceleration (the time interval between times t.sub.3 and t.sub.1 ) is chosen to restrict the desired acceleration to a low enough level so that, if PTO 1 actually accelerated at that rate, no damage to the PTO or to attached equipment would occur. For example, the predetermined amount of time may be 2 seconds. Depending upon whether the desired acceleration exceeds or is less than the actual acceleration at a given time, the clutch control system causes PTO clutch 3 to transmit, respectively, more or the same torque such that the actual acceleration approaches the desired acceleration.
FIG. 3A shows the time variation during the PTO engagement process of the actual and desired speeds of PTO 1 and the actual speed of engine 2 (or a fraction or multiple thereof, to account for gear reductions or augmentations occurring between engine 2 and PTO 1), and thereby illustrates a typical PTO acceleration (engagement) pattern using the above-referenced PTO clutch control system. From an initial time t.sub.0 until a time t.sub.1, PTO clutch 3 is not yet engaged and is providing no torque and so the actual speed of PTO 1 remains zero. After time t.sub.1, PTO clutch 3 is engaged; that is, the plates of the clutch have been sufficiently compressed so that the torque transmitted by the clutch is effectively proportional to the hydraulic fluid pressure applied to the clutch. Consequently, PTO 1 begins to receive torque through PTO clutch 3 from engine 2 in proportion to hydraulic fluid pressure controlled by the PTO clutch control system and begins to accelerate, as shown by the actual PTO speed curve. FIG. 3A also shows a desired PTO speed curve that has a constant slope reflecting the constant desired acceleration that would be necessary to cause PTO 1 to attain the measured initial speed of engine 2 (or the appropriate fraction or multiple thereof) within the predetermined amount of time (the time interval between times t.sub.3 and t.sub.1). As shown, the actual PTO speed typically lags the speed that would have occurred if PTO 1 consistently accelerated at the desired acceleration. However, though the actual acceleration of PTO 1 typically lags behind the desired acceleration of the PTO, it may exceed the desired acceleration as well. Throughout the process, the PTO clutch control system recalculates the actual acceleration of PTO 1 and adjusts the torque transmitted by PTO clutch 3 so that the actual acceleration approaches the desired acceleration.
Although the PTO clutch control system does allow for a controlled acceleration of PTO 1, the system inaccurately presumes that the speed of engine 2 remains constant during the PTO engagement process and therefore that a single measured angular velocity of input shaft 4 is an accurate basis upon which to calculate the desired acceleration. As shown in FIG. 3A, in practice this assumption is incorrect since the speed of engine 2 (and input shaft 4) typically droops as torque is transmitted by PTO clutch 3 from input shaft 4 to output shaft 10 (and then to PTO 1). That is, the speed to which output shaft 10 must accelerate in order to reach the speed of input shaft 4 is reduced. Assuming that the actual acceleration of PTO 1 is within a reasonable proximity of the desired acceleration, the time required for the PTO to reach the engine speed (or appropriate fraction or multiple thereof) therefore is reduced from the predetermined time used to calculate the desired acceleration (the time interval between times t.sub.3 and t.sub.1) to a shortened time, the time interval between a time t.sub.2 and time t.sub.1. Consequently, the acceleration and associated stress experienced by PTO 1 is larger than is necessary to accelerate the PTO to the engine speed (or appropriate fraction or multiple thereof) by predetermined time t.sub.3. It should be noted that the actual acceleration of PTO 1 is directly related to the magnitude of droop in engine speed and so, as one increases the desired and actual accelerations, one produces greater engine speed droop and exacerbates the above-described problems.
The above-described PTO clutch control system distinguishes between and responds to only two operational conditions (i.e., whether the actual acceleration exceeds or is less than a given desired acceleration). However, the control system may be modified to include a proportional adjustment algorithm wherein the degree of adjustment of the torque transmitted by PTO clutch 3 depends upon the degree by which the desired acceleration exceeds the actual acceleration. For example, the proportional adjustment algorithm may distinguish among three different levels of difference by which the desired acceleration may exceed the actual acceleration: if the actual acceleration of PTO 1 is less than the desired acceleration but greater than two-thirds of the desired acceleration, the control system increases the torque transmitted by PTO clutch 3 at a slow rate; if the actual acceleration is less than two-thirds of the desired acceleration but greater than one-third of the desired acceleration, the control system increases the torque transmitted at a medium rate; and if the actual acceleration is less than one-third of the desired acceleration, the control system increases the torque transmitted at a fast rate. Thus, the proportional adjustment algorithm causes the actual acceleration to approach the desired acceleration at a faster rate as the difference by which the desired acceleration exceeds the actual acceleration increases. As with the unmodified PTO clutch control system without the proportional adjustment algorithm, the modified PTO clutch control system may be configured to maintain the torque transmitted by PTO clutch 3 at a constant level if the actual acceleration exceeds the desired acceleration.
Although use of this modified PTO clutch control system provides for a more nuanced response to differences between the desired and actual accelerations, such use may also cause a second undesirable stress on PTO 1 (or attached equipment) when operating in conjunction with over-running clutch 6 as a result of non-ideal operation of PTO clutch 3, as described below. As described above, over-running clutch 6 only transmits power from input 8 to output 7 when locking pins 9 are engaged with locking notches 11. It is frequently the case that, before engagement of PTO 1, locking pins 9 are not engaged with locking notches 11 and, instead, input 8 is oriented relative to output 7 such that, for the locking pins to engage the locking notches, the input must rotate counter-clockwise a portion of a revolution with respect to the output. This is particularly the case since implement output shaft 13 may have rotated due to spurious movement of the attached equipment while PTO clutch 3 was disengaged (e.g., certain types of attached equipment may rotate due to contact with the ground as the agricultural vehicle and implement move forward, even though no power is being transmitted from engine 2 to the equipment via PTO 1). Also, locking pins 9 may disengage from locking notches 11 even when engine 2 is delivering power through PTO clutch 3 in circumstances where implement output shaft 13 (and the attached equipment) begins rotating at a rate that is faster than the rotational rate of PTO 1 (e.g., where engine speed is reduced). In either of these situations, it is possible for PTO 1 to receive power from engine 2 via PTO clutch 3 and to rotate (such that input 8 rotates counter-clockwise with respect to output 7) for a short period of time without transmitting any power to implement output shaft 13. This is because PTO 1 (and implement input shaft 5) must first rotate a portion of a revolution so that locking pins 9 engage locking notches 11 before implement input shaft 5 engages implement output shaft 13 through over-running clutch 6. In other words, there may be a "locking delay" in the response of implement output shaft 13 to power transmitted from engine 2 due to over-running clutch 6.
While the locking delay is of little concern at those times when PTO clutch 3 is engaged, the locking delay is problematic during engagement of PTO 1 as controlled by the modified PTO clutch control system. As described above, PTO clutch 3 is modeled as ideally having two distinct operational states, (a) a first, disengaged state in which the plates of the clutch are not compressed and so the clutch does not transmit torque between input shaft 4 and output shaft 10 (and then to PTO 1), and (b) a second, engaged state in which the plates of the clutch are compressed and the clutch transmits torque in an amount approximately directly related to the hydraulic fluid pressure applied to the clutch. However, in practice, PTO clutch 3 may still transmit a small but not negligible amount of torque from input shaft 4 to output shaft 10 even during the first, disengaged state, particularly if the hydraulic fluid pressure within the clutch is being increased to compress the plates and to cause the clutch to enter the engaged state. Even though this small amount of torque is typically insufficient to rotate implement output shaft 13 if the shaft is attached to equipment, the torque may be sufficient to rotate PTO 1 while locking pins 9 of over-running clutch 6 are disengaged from locking notches 11 and until such time as output 7 locks to input 8 (that is, the torque may be sufficient to rotate the PTO during the locking delay). To summarize, PTO clutch 3 may transmit enough torque from input shaft 4 to PTO 1 during the PTO engagement process, before the clutch is engaged, that the PTO will rotate from a position in which locking pins 9 of over-running clutch 6 are disengaged from locking notches 11 to the position in which the locking pins are engaged with the locking notches.
This rotation of PTO 1 before engagement of PTO clutch 3 results in undesirable consequences given the design of the modified PTO clutch control system, as shown in FIG. 3B (prior art). As described in U.S. Pat. No. 5,494,142, the PTO engagement process effectively begins when a PTO on/off switch (not shown) is closed by the operator of the agricultural vehicle. After that time, the PTO clutch control system (whether modified or unmodified) determines whether PTO 1 (i.e., output shaft 10) is rotating as an indication of whether PTO clutch 3 is engaged. Once PTO 1 is determined to be rotating, the PTO clutch control system (whether modified or unmodified) then begins to compare desired and actual accelerations, and begins to adjust the hydraulic fluid pressure applied to PTO clutch 3 in response to differences between the desired and actual accelerations in order to accelerate the PTO. While the design of the PTO clutch control system presumes that PTO rotation is a good indication of engagement of PTO clutch 3, as discussed above PTO 1 may begin to rotate before the clutch is engaged so long as the clutch transmits some torque and locking pins 9 of over-running clutch 6 are not engaged with locking notches 11. As shown in FIG. 3B, PTO 1 may have a nonzero speed between times t.sub.1 and t.sub.s, at which time PTO clutch 3 is engaged and controlled acceleration of PTO 1 begins, continuing until some later time, t.sub.L. Nevertheless, under these conditions the PTO clutch control system still senses the PTO rotation (between times t.sub.1 and t.sub.s) and consequently begins to compare the desired and actual accelerations and to adjust the hydraulic fluid pressure applied to PTO clutch 3. Because the actual torque transmitted to PTO 1 prior to the engagement of PTO clutch 3 is small (and, in any case, the PTO stops rotating upon the engagement of locking pins 9 and locking notches 11 of over-running clutch 6), the difference between the desired and actual accelerations immediately becomes large. The modified PTO clutch control system responds to this large differential (i.e., a differential in which the actual acceleration is less than one third of the desired acceleration) by increasing the hydraulic fluid pressure applied to PTO clutch 3 at a fast rate. Consequently, once PTO clutch 3 is engaged, PTO 1, implement input and output shafts 5 and 13, and any attached equipment immediately experience rapidly increasing torque and rapid acceleration along with related, undesirable stresses. (In contrast, the unmodified PTO clutch control system would respond to this large differential by slowly increasing the hydraulic fluid pressure applied to PTO clutch 3, thereby slowly increasing the torque transmitted by the clutch, and PTO 1 would not experience the extreme acceleration or overshoot in the valve command.) This effect may be exacerbated if, as in some systems, the fast rate of increase command for the hydraulic fluid pressure is faster than the hydraulics response time. If this occurs, the system may determine that the hydraulic fluid pressure should be increased even faster, and so the resulting command to increase the hydraulic fluid pressure may overshoot the optimum value and produce excessive acceleration of PTO 1.
It should be noted that, while the above situation involving over-running clutch 6 is the most common example of a circumstance in which spurious torque communicated through PTO clutch 3 may cause the modified PTO clutch control system to inappropriately increase hydraulic fluid pressure, this is not the only such circumstance. Any circumstance in which PTO 1 may begin rotation in response to spurious torque communicated through PTO clutch 3 when still not engaged may result in the same inappropriate responses.
Accordingly, it would be advantageous to develop an improvement for existing PTO clutch control systems that would enable the systems to control the engagement of a PTO so that the PTO would accelerate to attain the speed (or a multiple or proportion of the speed, depending upon gear reduction) of an engine both within and in not substantially less than a predetermined amount of time despite engine droop due to engagement of the PTO. It would also be advantageous to develop an improvement to the modified PTO clutch control system (as such system is described above) whereby the control system would avoid producing excessive acceleration of a PTO during PTO engagement even though (a) a PTO clutch acted in a non-ideal fashion to transmit torque before the clutch was engaged, (b) the PTO rotated in response to the transmitted torque until the locking pins of an over-running clutch engaged the locking notches of the over-running clutch, (c) the PTO clutch control system sensed the PTO rotation, and (d) the PTO clutch control system consequently began to compare desired and actual accelerations of the PTO and adjust torque transmission by the PTO clutch in response to the differential between those accelerations. It would further be advantageous if these improvements to the existing PTO clutch control systems could be implemented by making only minor changes to the existing PTO clutch control systems.